Apparatus for supplying AC current to an underwater cable

ABSTRACT

A device for burying a submarine cable under water by easily and accurately directing the cable position and a method for guiding the same by utilizing the characteristics unique to submarine cables, by supplying an AC only to the portion of the cable between two arbitrary repeaters through a so-called transformer-coupling system to generate an AC magnetic field, and by detecting the magnetic field with a magnetic field detector.

This is a division of application Ser. No. 282,876, filed July 13, 1981,now U.S. Pat. No. 4,540,942 issued Sept. 10, 1985.

BACKGROUND OF THE INVENTION

The present invention concerns a device for burying submarine cables orre-burying cables which are laid out on the sea and river beds, and amethod for guiding the same.

The cables for communication, electric power etc. in recent years aremore and more installed for longer distances and increasing number ofthe cables have been laid out upon sea or river beds. Since this type ofcable is required to offer a higher reliability, various methods havebeen devised for installation thereof; for instance, there has beenwidely used an installation method to bury the cable under the sea orriver beds in a fairly shallow water area in order to protect the cablefrom damage which might be caused by fishing gear or ship anchors. Formaintaining high reliability of the cable system it is of utmostimportance to have sufficient technology required to monitor the cableinstallation works, to repair it promptly at the time of failure and/orto give proper protection after such repair (such as re-burying worksfor the cables which have been excavated out of the bed for repair).Such technology has currently been the object of research anddevelopment in various countries.

In order to achieve proper works for monitoring, repair, protection,re-burying etc. for the cables, it is required to have the technology todetect the position of the cable which has been buried under the seabed, and to have cable tracking technology to move working machinesaccurately along the buried cable. This requires an extremelysophisticated technology of detecting the cable position especially whenthe cable to be detected has been buried or installed underneath the seabed in the open sea of a deeper water depth.

The monitoring method with an underwater TV camera mounted on a workingmachine has generally been used in prior art for detection or trackingof the submarine cables. However, that method is not necessarilysatisfactory since it cannot be applied to buried cables and furthersince its detection performance is greatly limited in the water of highturbidity. Another method for detection and tracking has been proposedto be effective under any conditions by supplying a signal current tothe cable and by detecting the magnetic field which is generated by thesignal current. The method using direct current (DC) as the signalsurrent, however, is not quite effective because of the difficulty todistinguish between the DC magnetic field generated from the cable andthe geomagnetic field. Therefore, the development of a method using AChas become urgently needed.

The submarine cable communication system using coaxial cables hasdrawbacks in that the desired frequency which will not affect thefrequency band to be used for communication is limited to a few hundredHz and that if such low frequency alternating current (AC) current issupplied from a terminal, the AC current becomes attenuated by theinternal power separating filters everytime it passes through arepeater/amplifier (referred to as repeater hereinafter) inserted on thepath to become almost zero after passing a few repeaters. Therefore, amethod to supply an AC signal current to the cable from a place otherthan the terminal is strongly demanded.

The present invention aims to obviate such defects of the conventionalmethod, and at the same time, to offer a device for burying thesubmarine cable under the water bed by easily and accurately detectingthe cable position and a method for guiding the same by utilizing thecharacteristics unique to submarine cables, by supplying an AC only tothe portion of the cable between two arbitrary repeaters through aso-called transformer-coupling system to generate an AC magnetic field,and by detecting the magnetic field with a magnetic field detector.

The method and the device according to the present invention will now bedescribed in more detail referring to the embodiments indicated inattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b) and 1(c) schematically explain an embodiment of themethod according to the present invention for generating AC magneticfields.

FIG. 2 is a schematic view of the state wherein AC signal currentpassing through the cable is being detected.

FIG. 3 is an explanatory view of the case wherein the method accordingto the present invention is applied to a disconnected cable.

FIGS. 4(a), (b) and (c) show a state wherein a diver is going to couplean annular closed circuit with a cable.

FIG. 5 shows another embodiment of the structure of the cable couplingdevice.

FIGS. 6(a), (b) and (c) are explanatory views to show a method forcoupling with, and recovering an annular closed circuit from, a cablewhich has been laid out on sea beds in water too deep for a diver toreach.

FIGS. 7(a) and (b) are explanatory views showing an embodiment of theannular closed circuit which is equipped to catch cables.

FIGS. 8(a) and (b) show a method for coupling an annular closed circuitwith a cable which has been lifted aboard the mother ship.

FIGS. 9(a) and (b) show one embodiment of the burying device used in thepresent invention; FIG. 10 is an explanatory diagram for a magneticsensor system, and FIG. 11 is an explanatory diagram for showing themaximum sensitivity detection directionality of the magnetic sensorsystem.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1(a) is a schematic view of an embodiment according to the presentinvention to explain a method for generating an AC magnetic field. Thesubmarine cable system using a coaxial cable comprises a cable 1 andrepeaters 2, 3 positioned at both sides of the cable. The cable 1comprises an inner electric conductor 4, an outer electric conductor 5,and an insulation cladding 6. The inner electric conductor 4 goes intopower separating filters 021,031 and is electrically connected torepeater casings 023,033 by DC circuits 022,032 the impedance of whichbecomes comparatively small at low frequency and is earthed through therepeater casings 023,033 to the sea water. The outer conductor 5, on theother hand, is insulated from the sea water in the cable portion by theinsulation cladding 6, but is connected to the repeater casings 023,033in the portions of the repeaters 2, 3 so as to be earthed directly tothe sea water therethrough. Accordingly, the following two closedcircuits which allow low frequency AC current to pass therethrough areformed in this system. More particularly, there are formed:

Closed circuit 1: outer conductor 5→repeater casing 023→seawater→repeater casing 033→outer conductor 5

Closed circuit 2: inner conductor 4→power separation filter 021→repeatercasing 023→sea water→repeater casing 033→power separation filter031→inner conductor 4.

Therefore, if an exciter coil having an exciter wiring is coupled insuch a way as to form an annular closed magnetic circuit at an arbitraryposition of the cable which encircles the cable axis, and if AC signalsfor excitation are supplied to the exciter wiring, then AC signalcurrent passes to the cable. The present invention utilizes such aprinciple, and by supplying AC signal current to the cable from anarbitrary position other than the terminal stations, an AC magneticfield is generated, which is then detected by the magnetic fielddetector of the burying device so as to guide the burying device alongthe cable.

FIG. 1(b) shows the state wherein a magnetic core 9 having an excitingwinding 8 is coupled to an arbitrary portion of a cable so as to form anannular closed circuit 7 encircling around the cable 1 in order tosupply an AC signal current to said two closed circuits. The above statecould be regarded as the state wherein the annular closed circut 7 formsa 4-winding transformer having four windings, i.e. that of excitingwinding 8, one turn of windings respectively formed by said two closedcircuits and one turn of winding formed by the sea water alone. When anAC current i₁ is supplied to the exciting winding 8 of the annularclosed circuit 7 via the oscillator 10 and the power amplifier 11,therefore, currents of i₂, i₃ and i₄ can be supplied to other threewindings depending on the turn ratio against the exciting winding 8 andthe impedance of the respective circuits. FIG. 1(c) indicates the stateshown in FIG. 1(b) in a form of an equivalent circuit for betterunderstanding, whereby the portion shown by a thick line denotes "thecurrent line" where current passes through the sea water. While thecurrent i₄ can not be utilized because it does not pass through thecable, the currents i₂ and i₃ can be utilized as the signal currentbecause they pass through the cable. The above i₂, i₃ currents can beused for such purposes as to guide the travel of a working machineaccurately along a cable by tracking the position of the cable bydetecting the signal current by, for instance, a magnetic field detectorcarried on the machine. The impedance Z₃ in the closed circuit 1 shownin the above figure is infinitive in respect of DC and becomes fairlylarge value in respect of low frequency, whereby it becomes larger thanthe impedance Z₂ in the closed circuit 1. Therefore, it could be saidthat the current i₂ plays a main role in the present invention.

FIG. 2 schematically indicates the concept in which the AC signalcurrent i₂ passing through the cable can be detected. In the systemaccording to the present invention, since the return circuit of thesignal current i₂ passing through the cable is formed by the sea waterwhich is extensive in three dimensions, there arises a doubt whether thesignal current i₂ can be effectively detected by an AC magnetic fielddetector which is set up within the sea water. However, as shown in FIG.2, the current on the return circuit does not generally concentrate onthe water immediately near the cable but passes divergently as well asextensively to reach the mass of water considerably apart from the cablein respect of a low frequency or a frequency smaller than several KH_(z)except for the case of fairly high frequency. Therefore, in the case ofthe detector D₁, the magnetic field out of the current i₂ passingthrough the cable can be effectively detected because the annular closedcircuit is adapted to be positioned very close to the cable so that thecurrent which passes through the inner side of the annular closedcircuit as the current effective for the detection by the means of thesea water can be offset by the current which passes through the outerside to the direction to cancel the detection signals. In the case ofthe detector D₂, on the other hand, the current i₂ passing through thecable can be effectively detected by such measures as to prevent the seawater from passing through the inside of the annular closed circuit byminimizing the diameter of the annular closed circuit.

In order to verify such phenomena as mentioned above, an experiment wasconducted wherein a submarine coaxial cable of about 20 m in length wasused in the sea water in the construction shown in FIG. 2. In theexperiment, a Senper Max material of 100×60×20 mm is used as a core ofan annular closed circuit, the number of windings of the excitingwinding N₁ is 20 turns, V₁ =2.7 V and i₁ =23 mA. When it is excited with1 KH_(z), a current of 29 mA is obtained as the cable signal current i₂.In this case, the impedance of the current loop constructed with thecable and the sea water was measured to be 5Ω, and when such values areexpressed in the theoretical formula indicated below,

    i.sub.2 =(V.sub.1 /N)·(1/R)                       (1)

i₂ =27 mA is obtained to be considerably approximate to the measuredvalue.

FIG. 3 schematically explains that a signal current can be supplied tothe cable (according to the method of the present invention) even whenthe submarine cable has been cut by fishing gear etc. In the figure,since the outer conductor 5 becomes exposed to the sea water at thebroken point, the outer conductor 5 comes to have a return circuitcomprising the sea water so that the signal current i₂ can be passed tothe outer conductor 5 by supplying an exciting current i₁ to theexciting winding 8.

To achieve the purpose of the present invention, said annular closedcircuit 7 has to be coupled with a submarine cable 1, but the methodvaries depending on the conditions of the submarine cable. FIG. 4indicates a method wherein a diver 12 couples an annular closed circuit7 with a cable 1 which has been laid out beneath the sea at such ashallow depth that a diver can reach it. FIG. 4(a) shows that after thediver has coupled two magnetic substances separated into twosemispherical bodies 9₁, 9₂ with the cable 1, an AC signal current i₁ issupplied to the exciting winding 8 via a lead line 14 from an oscillator10 and a power amplifier 11 mounted on the mother ship 13. In the figurethe reference numeral 15 denotes a locking device made of a magneticsubstance used for securely fixing the two magnetic substances 9₁, 9₂ toform an excellent annular closed circuit 7.

FIG. 4(b) indicates another method wherein the method for coupling theannular closed circuit 7 with the cable 1 is identical to the one shownin FIG. 4(a), but the oscillator 10 and the power amplifier 11 forexciting the exciting winding 8 are housed within a buoyant water-tighthousing 16 to be used as an AC signal current supply device (hereinafterreferred to as a supply device) and the supply device is positionedclose to the annular closed circuit 7. The reference numeral 17 denotesa battery. In this method, in order to prevent the power loss caused bythe curret i₄ as indicated in FIG. 1(c) as the sea water flows throughthe inner side of the annular closed circuit 7, a diaphragm wall 18 maybe provided as shown in FIG. 4(c).

Both in the embodiments shown in FIGS. 4(a) and (b), since an AC signalcurrent i₂ passes to the cable under the condition indicated, theposition of the cable can be detected by detecting the magnetic fieldcreated by the current i₂ with a magnetic field detector, so thatoperational machines etc. may be enabled to perform monitoring ormaintenance work. In both cases, when the operation ends, the diver 12detaches the lock 15 and recovers the magnetic substances 9₁, 9₂ aboardthe mother ship 13. In the present embodiment, although description wasmade in respect of a structure where a water-tight housing having abuoyance was used as a guide signal supplying device, the presentinvention is not to be limited to this embodiment and the system may beof any arbitrary type such as one which directly supplies from themother ship, or which is incorporated inside a cable coupling device towhich reference will be made later.

FIG. 5 shows the structure of another embodiment of the cable couplerwherein an annular closed magnetic circuit type exciter coil 9 wouldwith an exciter wiring 8 and a battery 17 are sealed inside anon-metallic (i.e. plastic) water-tight housing 16 of the co-axialstructure. When the guide signal supplying device as shown in FIG. 4(b)is used, AC signals for excitation are supplied to the exciter coil 9 bythe lead cable 14, and AC signal current is passed to the cable coupledto the axial core of the water tight container based on the principlediscussed before.

FIGS. 6(a), (b) and c explain the method for coupling an annular closedcircuit 7 with a cable which has been buried underneath the sea bed inwaters of a depth which a diver can not reach and/or detach it from thecable so as not to obstruct operations.

FIG. 6(a) indicates a state wherein an annular closed circuit 7 in ashape like a plough having a cable-grasping function is towed upon thesea bed by a front-tow cable 19 connected to the mother ship 13 and moreparticularly a state wherein the cable 1 is about to be grasped by theplough 20. The reference numeral 21 denotes a weight which stabilizesthe travelling of the annular closed circuit 7, 22 a rear-tow cable, and23 a buoy which gives the rear-tow cable a small degree of buoyancy.

FIG. 6(b) indicates a state wherein, after the annular closed circuit 7has grasped the cable 1, the closed circuit 7 is coupled with the cable,supplying an AC signal current to an exciting winding 8 of the annularclosed circuit 7 via the signal lines within the rear-tow cable 22 fromthe exciting power source 24 comprising an oscillator, a poweramplifier, a battery etc. to pass the AC signal current i₂ through thecable 1. Since in this method the exciting power source 24 is separatedfrom the mother ship 13, the mother ship 13 can concentrate its effortson the controlling of the working machines 25. The reference numeral 26denotes buoys to keep both two cables 19 and 22 afloat on the watersurface. FIG. 6(c) indicates a state wherein the annular closed circuitis being detached from the cable 1 in order to remove it out of the wayof the operation or after the operation and is to be recovered by themother ship 13 or a support ship 13'. The buoy 26 to hold the rear-towcable 22 and the exciting power source 24 which are used in FIG. 6(b)can be collected to, for instance, a support ship 13' and the annularclosed circuit 7 is detached from the cable 1 by pulling the rear-towcable 22 backward.

FIGS. 7(a) and (b) show an embodiment of the annular closed circuit 7having a cable grasping function.

FIG. 7(a) corresponds to the annular closed circuit 7 described in FIG.6 which is equipped with the function to grasp cables. In the figure,the symbols 9₁ and 9₂, denote magnetic substances constructing anannular closed circuit 7 as they are closed and 9₂ has an excitingwinding 8 and is fixed on the base of the plough 71 while 9₁ is amovable magnetic substance connected to a helicoid straight drivingmeans 73 which is driven by a motor 72. As the cable 1 is grasped by theplough 71 and brought to the position indicated by a dotted line in thefigure, the inductance changes on the exciting winding 8. Since the leadline of the exciting winding 8 extends through the rear tow cable to abus bar, the changes in the inductance can be immediately detected fromaboard the mother ship so as to know that the cable 1 has come into themagnetic substances 9₁, 9₂. Then, the motor 72 is directed from themother ship to start lowering the movable magnetic substance 9₁ so as tobring it into contact with 9₂, completing an annular closed circuit 7.Load cells 74 are inserted between the annular closed circuit 7 and thefront and rear tow cables 19, 22 to measure towing tensile strength;therefore, whether the cable 1 has come to settle in a predeterminedposition or not can be detected by measuring the output from the loadcell 74, too. The annular closed circuit 7 can be detached from thecable 1 simply by following the above procedure in the reversed orderand more particularly, by moving the helicoid straight driving means 73to move the movable magnetic substance 9₁ upward and then by pulling theannular closed circuit 7 rearward with the rear-tow cable 22.

FIG. 7(b) shows a still other embodiment of the annular closed circuit 7equipped with the function to catch cables. In the embodiment, theannular closed circuit 7 is adapted to include a power supply means forthe exciting power source 24 comprising an oscillator 10, a poweramplifier 11 etc. It further is adapted to have a cable sensor 75 suchas, for instance, a metal detector, which is embedded in the plough 71in order to detect whether the cable 1 has been grasped by the plough 71or not with a higher accuracy. The reference numeral 76 denotes anelectronic circuit of the cable sensor of a metal detector type 75wherein a detection signal obtained by the circuit 76 is transmitted tothe mother ship via the rear-tow cable 22. The annular closed circuit 7shown in FIG. 7 is applicable to the cable laid out upon sea beds or tothe cable buried therein.

FIGS. 8(a) and (b) indicate a method for coupling an annular closedcircuit 7 with a cable 1 aboard the mother ship 13 when the cable 1 islifted aboard the ship to repair disconnected portions. FIG. 8 (a) showsa state in which the cable 1 held by the mother ship 13 has been coupledwith the annular closed circuit 7 and is supplied with the current fromthe oscillator 10 and the power amplifier 11 so as to pass an AC signalcurrent i₂ to the cable 1. FIG. 8(b) depicts a situation wherein theannular closed circuit 7 which has been coupled with the cable 1 on themother ship 13 as shown in FIG. 8(a) is thrown into the water togetherwith the exciting power source 24 comprising the oscillator 10, thepower amplifier 11, the battery 17 etc. The reference numeral 16 denotesa water-tight housing and a means to detach the annular closed circuit 7from the water-tight housing 16 containing the exciting power source 24by instructions sent via ultrasonic signal after the cable has beenconnected to the machine so that the housing would not be in the wayduring a protection operation such as burying the cable.

The method for coupling an annular closed circuit 7 with the cable 1aboard the mother ship 13 is extremely effective for the cableinstallation construction work for the cable system. In this method thecable 1 is thrown into the water from the mother ship 13 consecutively.Since an unburied portion of the cable is always kept aboard the mothership, the coupling of the cable with the annular closed circuit 7 isreadily conducted to supply a signal current to the cable 1. A monitordevice for the construction can be made to travel along the cable simplyby detecting an AC magnetic field generated by the signal current.

FIGS. 9(a) and (b) show one example of a structure for the buryingdevice. The burying device 25 has a travelling crawler 251 driven by thepower supplied from the mother ship via the power line inside thecontrol cable 259, and supplies the jet water to the injection nozzle524 inside the injection excavation mechanism 253 by using a water bedexcavation means disposed on the burying device 25, for instance, aninjection pump 252, thereby removing the earth around the cable andburying the cable. The burying device 25 is further provided withmagnetic sensor systems 255,256, and is so made that it can detect themagnetic field generated by the current i₀ which is given by the formula(1) and passes through the cable 1. Supposing that the current i₀ passesto the cable, the intensity of the magnetic field (H₀) located at apoint L (m) away from the cable becomes approximately

    H.sub.0 (gamma)=(200 i.sub.0 /L)                           (2)

Thus, by guiding the burying device 25 toward the direction where thedetected magnetic field magnifies, it is possible to easily bring theburying device 25 closer to the cable 1 which is to be buried.

Reference is made to FIG. 10 in order to explain the structure of themagnetic sensor systems 255,256. The sensors comprise coils 2551,2552and coils 2553,2554 respectively positioned perpendicular to each other,the two sets of coils being placed inside the water tight housing 2555with an interval of l therebetween, and each of the coils has itsmaximum sensitivity in the axial direction of the coil. In other words,the coils 2551 and 2553 have the maximum sensitivity detectiondirectivity in the horizontal direction, and the coils 2552 and 2554 inthe vertical direction. As shown in FIG. 11, supposing that the coils2551,2552 and the coils 2553 and 2554 are at the positions P₁ and P₂,and the cable 1 is positioned at a point lower by h from the plane of P₁-P₂, then there accrue magnetic fields having the directions H_(P1) andH_(P2) at points P₁, P₂ by the current i₀ passing inside the cable fromthe direction of the right face of the paper toward the back thereof asshown in FIG. 11. When the distances between the cable 1 and the pointsP₁, P₂ where the coils are positioned are set respectively as L₁, L₂,then the magnetic fields H_(P1), and H_(P2), in the vertical directionof the magnetic fields H_(P1), H_(P2) at the points P₁, P₂ where thecoils are positioned are sought by the following formulae. ##EQU1##Where the cable 1 is over the center line of the line between the pointsP₁ and P₂, the above formula (3) obtains

    L.sub.1 =L.sub.2, θ.sub.1 =θ.sub.2,

and H_(P1) ' and H_(P2) ' become magnetic fields which are equal to eachother in size, but reverse in symbols.

From the above, it becomes apparent that the burying device 25 is guidedto a point right above the submarine cable 1 which is to be buried bymoving the travelling crawler 251 of the burying device 25 in such a waythat the components in the vertical directions to be detectedrespectively by the magnetic sensor systems 255,256 (FIG. 9) have theconstruction shown in FIG. 10 would become the same size but of theopposite symbols after the burying device 25 approaches the submarinecable 1.

On the other hand, the tracking signal supplying device 257 of a cableguide system shown in FIGS. 9(a) and (b) catches the cable 1 with an armcomprising a portion of the supplying device 257 after the buryingdevice reaches a position immediately above the cable 1, and theninserts an annular type closed magnetic circuit exciter coil having theexciter wiring of the identical structure as shown in FIG. 5 over thecable thus caught. The exciter coil is provided integrally with saidcable catching arm on the supplying device 257, and becomes separatedinto two when being inserted over the cable, and closed thereafter tobecome integral with the cable 1 and form a closed magnetic circuit.After being inserted over the cable 1, the exciter coil is used tosupply AC signals to the submarine cable 1 for guiding the buryingdevice, while the guide signal supplying device of FIG. 4, which hadinitially been attached to the cable 1 at the portion denoted 16 of thewater tight container shown in FIG. 4, is cut off upon receipt ofinstruction signals from an ultrasonic oscillator 258 provided in theburying device 25 by the receiver inside the cutting device since thesupplying device gets in the way of the burying operation.

The tracking signal supplying device 257 of a cable guide type shown inFIGS. 9(a) and (b) catches the cable 1 by its arm which forms a partthereof after the burying device 25 moves immediately above the cable 1,and inserts the annular closed magnetic type exciter coil having anexciter wiring of the identical construction as shown in FIG. 5 over thecable 1 thus caught. The exciter coil is provided on the supplyingdevice 257 integrally with said arm, and becomes separated when beinginserted, and then integrated into one after insertion over the cable 1,thereby forming a closed magnetic circuit. The exciter coil is used tosupply AC signals for guiding the burying device 25 to the cable to beburied 2 after the coil is inserted over the cable 1. The guide signalsupplying device of FIG. 4, which had been attached initially to thecable 1, at the portion 16 of the water tight container is cut off uponreceipt of instruction signals from an ultrasonic oscillator 258provided on the burying device 25 by the receiver inside the cuttingdevice since the supplying device gets in the way of the buryingoperation. In the present embodiment, explanation was given in respectof the system where an exciter coil of an annular closed magneticcircuit with an arm for catching the cable being integrally built isused to guide the burying device 25 along the cable 1 after engaging theburying device 25 with the cable 1. The present invention is not to belimited to this embodiment alone, and it may utilize the cable couplingdevice (of FIG. 5) which had been used to bring the burying device 25closer to the cable 1 prior to their engagement. In this case, acatching means for the cable coupling device may be provided integrallywith the cable catching means provided on the supplying device 257. Themagnetic sensor system 256 shown in FIG. 9 is used along with themagnetic sensor system 225 for detecting the positions of the buryingdevice 25 and the cable 1. It is also used to detect the depth of theburied cable 1. The distance h between the cable 1 and the sensor plane(the plane connecting P₁ and P₂) when the cable 1 and the sensors (atpositions P₁, P₂) are in the relative positions as shown in FIG. 11 maybe sought by the formula ##EQU2## wherein tan θ₁ and tan θ₂ are given bythe ratio of the detection outputs of the coils 2551 to 2554 of asensors, viz. ##EQU3##

As has been discussed in detail referring to the embodiment disclosed inthe drawings, it is possible to supply the signals from an arbitraryportion of the submarine cable for guiding the burying device accordingto the present invention. Thus, the burying device can approach thecable accurately while detecting the guide signals, and after havingapproached the cable, the burying device itself supplies the signals forguiding the burying device along the cable, while detecting the guidesignal by the magnetic field detecting device provided thereon foraccurate travelling of the device along the cable. Accordingly, thepresent invention offers a technology for performing a speedier andsecurer burying operation for the submarine cable and has a technologyof high inventiveness.

We claim:
 1. An apparatus for supplying an AC current signal to a cableunder water, comprising: a buoyant water-tight non-metallic container; abattery, an oscillator, and a power amplifier arranged in saidcontainer; said battery being electrically connected to said oscillatorto supply DC power thereto, said oscillator being electrically connectedto said amplifier for supplying AC current thereto; an annulus ofmagnetic material placed around said cable so as to leave an annularspace therebetween; an exciting winding placed around said annulus andelectrically connected to said power amplifier for supplying saidwinding with an AC current and thereby induce an AC signal in the cableby electro-magnetic induction, and an electrically non-conductingannular diaphragm wall closing said annular space to prevent power lossby flow of water through said space.